[Greenbuilding] Fresh air to boiler questions

Sat Jan 17 15:49:27 CST 2015

If you do not use direct air feed such as the 6” duct to boiler in this example, you will be counting on the equivalent clear area from random cracks in the thermal envelope.  On most common practice houses, this may seem like a reasonable assumption.  However, as the houses get tighter, this draw will depressurize the house and have more difficulty drafting, creating opportunity for backdrafts and CO, smoke entering the thermal envelope.  The duct for direct feed to combustion appliances in a closed loop is not a hole in your thermal envelope if it remains tight and separate from the air in the conditioned space.  A combustion appliance is required to loose some heat to vent toxic fumes.  Eliminating the vulnerability to back drafting is a huge benefit to closed loop combustion appliances.  The newer more efficient equipment I think have a forced draft that does not rely on the chimney affect of heat rising to vent.  I think this allows the combustion gases to go through some heat exchanger and therefore improve the efficiency. The 6” hole of the air feed vent is only a hole for conductive heat loss, not the higher convective heat loss of a hole that is open from outside to the conditioned air.  I suppose one could insulate the outdoor air feed duct up to the combustion appliance to reduce the conductive heat loss.  

Eli 

From: Norbert Senf 
Sent: Wednesday, January 14, 2015 4:04 PM
To: Green Building 
Subject: Re: [Greenbuilding] Fresh air to boiler questions

The air consumption of an 80,000 BTU/hr boiler is in the range of 25 cfm while it is operating. Heating that combustion air by 50 degrees F requires about 3,000 BTU/hr, which happens no matter whether the outside air has a direct or indirect path to the burner.  Having a 6" dia permanent leak in the basement is a really inefficient way to do it. There is no good engineering reason to have that leak, unless your basement is hermetically airtight and 25 cfm would depressurize it enough to cause the boiler or water heater or (worst case) an open fireplace to backdraft. This is generally considered to be -5 Pa. Much more likely that a 250 cfm kitchen exhaust hood would do it, assuming that your basement is not sealed from the rest of the house. Clother dryers and bathroom fans also, are in the 100 cfm range.

On Tue, Jan 13, 2015 at 7:43 PM, Benjamin Pratt <benjamin.g.pratt at gmail.com> wrote:

  I have been sealing up and insulating the unfinished portion of my basement, but am concerned it is a waste of time and money: There is a 6" fresh air duct to provide proper combustion for the boiler. Also, there is no damper on the water heater or boiler. Therefore, a lot of cold air is continually dumped into the basement, and a lot of hot air escapes up the chimney.
     I suppose the solution to this would be to install a new boiler, and to install a direct vent kit on the water heater. The boiler is around 15 years old, and is 84 percent efficient. If I buy a more efficient boiler, that is direct vented, any guess as to how much energy I will save? I suppose its more then just the amount of natural gas I will save with a more efficient boiler, since I would also be eliminating a huge source of air infiltration. I would like to be able to calculate the ROI, so my wife agrees to the purchase.
  I'd appreciate any help you can provide.
  Thanks,
  Ben

  b e n j a m i n p r a t t 

  professor art+design
  the university of wisconsin stout

  On Tue, Jan 13, 2015 at 3:46 PM, Sacie Lambertson <sacie.lambertson at gmail.com> wrote:

    I'm looking for an architect in Seattle who understands energy efficient good building and does contemporary design work.  Welcome suggestions.   Best to send these off the forum, to me directly. Sacie

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-- 

Norbert Senf
Masonry Stove Builders
25 Brouse Road, RR 5
Shawville Québec J0X 2Y0
819.647.5092
www.heatkit.com

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